1. Field of the Invention
The present invention relates to an outboard motor.
2. Description of the Related Art
Outboard motors provided with catalysts for improving environmental performance have recently been developed. For example, in the outboard motor disclosed in JP-A-2010-242744, a catalyst is disposed to the side of an engine. The engine includes a plurality of cylinders and a plurality of exhaust ports. The cylinders are aligned vertically. The exhaust ports are aligned vertically. The exhaust ports are connected to an exhaust manifold. An exhaust pipe storing the catalyst is connected to the exhaust manifold. Exhaust from the exhaust ports is collected in the exhaust manifold and sent to the catalyst. NOx and other contaminants included in the exhaust are cleaned by the catalyst.
To improve the catalyst cleaning performance described above, it is important that the flow of exhaust to the catalyst be uniform. In the exhaust catalyst device disclosed in JP-A-2007-211663, the flow of exhaust is made uniform by forming a swelling portion in the exhaust pipe. The swelling portion is formed in the exhaust pipe disposed between the exhaust ports of the engine and the catalyst. The swelling portion is formed in a wall surface of the exhaust pipe where exhaust emitted from the exhaust ports strikes at a substantially perpendicular angle. The swelling portion is configured so as to swell toward the outside of the exhaust pipe.
In an outboard motor, the engine is disposed in a space that is limited to only the interior of a cowling. Accordingly, there is only a very small degree of freedom to arrange the exhaust pipe connected to the engine. Furthermore, the degree of freedom provided to arrange the exhaust pipe is further reduced in cases in which the catalyst is disposed in the exhaust pipe in order to improve environmental performance. Therefore, it is difficult in an outboard motor to form a swelling portion such as the one disclosed in JP-A-2010-242744 in the exhaust pipe because there is insufficient space upstream of the catalyst.